When it comes to brewing 1 gallon of beer, the amount of yeast you need can vary depending on a few factors. The general guideline is to use about 2 grams of yeast per gallon for a decent fermentation. However, if you want to ensure a strong and healthy fermentation, using a third of the package, which is roughly 8 grams per liter, can provide some additional insurance.
Freshness of the yeast also plays a role in determining the amount needed. If you are using fresh yeast, 2 grams per gallon should be sufficient. Fresh yeast is highly active and can easily ferment the sugars in your beer. However, if your yeast is not as fresh or if you're unsure about its viability, using a larger pitch rate, such as a third of the package, can help ensure a successful fermentation.
It's worth noting that the desired alcohol content of your beer can also influence the amount of yeast required. If you're brewing a beer with an ABV (alcohol by volume) above 7%, it's generally recommended to use a higher pitch rate to handle the increased fermentation demands. In such cases, using a third of the package for 1 gallon would be a more comfortable amount.
To summarize, if you have fresh yeast and are brewing a beer with a moderate ABV, using 2 grams of yeast per gallon should be sufficient. However, if you want to be on the safe side or if you're brewing a higher ABV beer, using a third of the package, which is roughly 8 grams per liter, would provide better insurance for a successful fermentation.
Please note that these are general guidelines and may vary depending on the specific yeast strain and brewing conditions. It's always a good idea to consult the yeast manufacturer's instructions or seek advice from experienced brewers for more precise recommendations.